Genetic studies on the immune response continue to employ antibodies to streptococci which are produced in inbred rabbits. A unique feature of these antibodies is their remarkable molecular uniformity. Because of this uniformity, these antibodies are especially useful in examining the relationship among the genes which control antibody synthesis, and in exploring the nature of the mechanisms responsible for antibody diversity. For example, the detection of a similar molecular marker on the streptococcal antibodies from inbred rabbits and its absence among unrelated rabbits emphasizes the genetic control over antibody synthesis. A preferential association between the genes for this molecular marker and other genes points to multiple gene interactions in the synthesis of a specific antibody. Substantial progress was made in the following areas: a. the continuation of studies that trace the inheritance of idiotypic cross reactive determinants, and to determine the special associations between idiotype and allotype as well as the reasons for the occurrence of deviations from this association; b. continuation of a major effort to sequence additional homogeneous antibodies, paying particular attention to the variable regions of the light chains and the heavy chain, with the end in view of determining the structural correlates of idiotypy and allotypy; c. an investigation of the chemical structure of the streptococcal carbohydrates, the antigens which stimulate the occurrence of the homogeneous antibodies in rabbits and the development of a radioimmunoassay for the detection of antibodies to peptidoglycan in human sera, including those from patients with rheumatic fever and juvenile arthritis. BIBLIOGRAPHIC REFERENCE: Krause, R.M., Kindt, T.J., Bokisch, V.A., and Greenblatt, J.J. The antibody diversity following immunization with hemolytic streptococci. In the Streptococcal Disease and the Community. Amsterdam 1975, pp. 102; Bernstein, D., Klapper, D.G. and Krause, R.M. Use of Radioimmunoassays to Determine the Concentration of Streptococcal Group-Specific Antibodies In Rabbit Antisera. J. of Immunol., 114, 59, 1975.